A push-to-talk service (PTT) is an instant communication service, such as a radio service, and is intended to provide rapid communication. When compared with a general mobile communication service, PTT is highly desirable because a user can communicate with another party by pressing a talk button of a terminal without having to undergo through unnecessary processes such as dialing, a call-connection tone or the like. Also, PTT allows user voice and data communication to a single recipient (1-to-1) or between groups of recipients as in a group chat session (1-to-many).
Recently, a push-to-talk over cellular service (PoC), wherein the PTT service is applied to a mobile terminal, has increasingly drawn attention. Accordingly, development of a PoC terminal, a PoC server and a communication standardization, for example, are actively ongoing.
The PTT service system comprises a PTT terminal having a PTT client therein for calling a PTT service. The PTT service system also comprises a PTT server for establishing sessions between PTT terminals and controlling transmission of voices and data between the PTT terminals to thereby implement a variety of PTT services.
To implement the PTT service and take a talk burst, a first PTT terminal establishes a session with a second PTT terminal participating in the PTT service through the PTT server. Specifically, the first PTT terminal establishes a session with the PTT server by transmitting or receiving a session initiation protocol (SIP) message (INVITE, 200 OK). The PTT server then establishes a session with the second PTT terminal by transmitting or receiving a SIP message (INVITE, 200 OK) with reference to the INVITE message transmitted from the first PTT terminal.
The PTT server transmits a talk burst confirmation response to a PTT terminal requesting the talk burst to confirm permission to send the talk burst. The PTT server also transmits a talk burst reception indication to all PTT terminals, except the PTT terminal having permission to send the talk burst, to indicate an identity (ID) of the PTT terminal having the permission to send the talk burst.
Voice and data transmitted through the PTT terminal by a user is transmitted to another PTT terminal via the PTT server as a real-time transport protocol (RTP) packet. In case a talk burst transmission from the PTT terminal having permission to send the talk burst is finished, the PTT server transmits a “no talk burst indication” to all PTT terminals participating in the session. This indicates that no terminal has presently requested permission to send a talk burst. Presently, the RTP is an Internet protocol (IP) for transmitting data directly, and is generally used to transmit voice and image data on a network.
The PTT terminal should be synchronized with the PTT server in order to maintain a session between the PTT terminal and the PTT server, and finish the session. In the related art, the PTT terminal and the PTT server are synchronized with each other using a session time of the SIP. The PTT terminal transmits a SIP message to the PTT server to establish a session or transmits a SIP message (Update) to the PTT server before a session time is over to maintain a session between another PTT terminal that participates in the service. However, since the SIP message has a large size, the session time is not set to be short in length. Preferably, the session time may be set anywhere from a few minutes to tens of minutes.
In a related art synchronization method, in case that a PTT terminal is positioned in a shadow area of service (area of no radio coverage) and thereby service is no longer maintained, the PTT server can not sense a state of the PTT terminal for the session time. Accordingly, the PTT terminal continuously transmits the RTP packet to the PTT server.
In the related art synchronization method, the PTT terminal and the PTT server are not synchronized for the session time and thereby generate unnecessary traffic. Also, since the PTT server provides service to the PTT terminal even after the PTT terminal ends the service, the PTT terminal cannot perform another calling.
If a PTT terminal having a talk burst is moved to a shadow area of service, the PTT server confirms that an RTP packet is not received from the PTT terminal for a certain time and thereby confirms that the PTT terminal has deviated from a service area. However, if a PTT terminal, not having the talk burst but is listening to the talk burst, is moved to the shadow area of service, the PTT server can not confirm whether that PTT terminal has deviated from a service area until the session time is over. In this case, even if the user has released a session of the terminal moved to the shadow area of service, other terminals participating in the session cannot recognize the state of the terminal. Accordingly, a misunderstanding between users is caused, and service quality is degraded.
Therefore, in the related art synchronization method, the PTT server can not efficiently manage the PTT terminal. Thus, traffic is continuously generated at a PTT terminal, which has already released its session, thereby wasting network resources.